<?xml version="1.0" encoding="UTF-8"?>
<overlay xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
<box id="b">
<box onDOMAttrModified="event.target.parentNode.removeChild(event.target)" id="d"/>
<tree/>
</box>

<tree>
<box  id="b" observes="d"/>
<treechildren observes="b"/>
</tree>
</overlay>